The measurement of intracranial pressure (ICP) is important in diagnosing and treating various life threatening conditions caused by trauma, hemorrhage, tumors, inflammatory diseases and the like. Several techniques have been used to measure ICP. Conventional invasive ICP measurement techniques require a surgical passage through the skull bone into the ventricles, parenchyma or the region between the skull and dura mater to implant a measurement device.
A non-invasive ICP measurement technique has been suggested that determines distortions of the tympanic membrane of the ear. However, it has not been possible to obtain a good correlation with ICP because determination of ICP by this method is complicated by the compressible air space between the pressure source and the interrogation point.
Another non-invasive ICP measurement method measures the electro-magnetic impedance response of the brain to induced fields, and correlates the response to ICP. Such electro-magnetic measurement techniques are disclosed in U.S. Pat. Nos. 4,690,149 and 4,819,648 to Ko.
Another non-invasive ICP measurement technique that has been attempted involves ultrasonic imaging to detect relative displacements of tissue boundaries within the brain. The displacements may be associated with fluid build-up and compression or dilation of brain components, which permits determination of ICP through an independent calibration of compressibility. An alternate non-invasive ultrasonic technique involves the measurement of blood flow in the carotid artery by ultrasonic excitation of the artery and determination of Doppler frequency shift.
Various types of ultrasonic ICP measurement techniques are disclosed in U.S. Pat. Nos. 3,872,858 to Hudson et al., 4,043,321 to Soldner et al., 4,971,061 to Kageyama et al., 4,984,567 to Kageyama et al., 5,388,583 to Ragauskas et al. and 5,411,028 to Bonnefous. Such techniques involve the transmission of ultrasonic waves typically having frequencies on the order of 5 MHz into the cranium. A problem with ultrasonic excitation is the high intensities required in order to penetrate enough of the brain to sense the effect of increased ICP. Waves travelling through the intracranial region are absorbed at a substantially increasing rate as the frequency of the waves is increased. Ultrasonic frequencies on the order of 5 MHz require significant input power in order to produce usable signal-to-noise ratios. While ultrasonic input powers do not pose a health risk over relatively short time periods required for a typical clinical ultrasound scan, the FDA has limited cumulative exposure to 50 J/cm.sup.2. A 100 W/cm.sup.2 transducer utilizing one hundred 1 microsecond pulses every minute would exceed this limit in less than 100 hours. However, in some cases, patients may require continuous monitoring for at least a week. In addition, ultrasonic equipment is relatively large and expensive and is not suitable for field use or for dedicated monitoring of patients over long periods of time.
Each of the patents cited above is incorporated herein by reference.
Despite the above-noted attempts to develop ICP measurement techniques, no clinically useful devices are available and a need still exists for an ICP measurement apparatus and method which can measure ICP without skull penetration, which poses little or no health risks during long-term monitoring, and which is simple, compact and relatively inexpensive.